In general, a wheel position detecting device detects a position of a subject wheel in a vehicle. Also, it is known to use a wheel position detecting device in a tire pressure detecting apparatus for detecting a tire pressure of a wheel. As an example of the tire pressure detecting apparatus, a direct-type tire pressure detecting apparatus has been known.
In the direct-type tire pressure detecting apparatus, a transmitter is directly fixed to a wheel with a tire. The transmitter is provided with a sensor, such as a pressure sensor. A vehicle body is equipped with an antenna and a receiver. When the transmitter transmits a detection signal of the sensor, the receiver receives the detection signal through the antenna, and detects the tire pressure of the wheel based on the detection signal.
In such a direct-type tire pressure detecting apparatus, data transmitted from the transmitter includes ID information for identifying whether the transmitted data relates to a subject vehicle to which the tire pressure detecting apparatus is equipped, and for identifying the wheel to which the transmitter transmitting the data is fixed.
To specify the position of the transmitter based on the ID information of the transmitted data, the receiver stores beforehand a relationship between the ID information of each transmitter and the position of each wheel. When the positions of the wheels are changed by a tire rotation, it is necessary to register the relationship between the ID information of each transmitter and the position of each wheel again. For example, a patent literature 1 describes a tire pressure monitoring apparatus and a tire pressure monitoring method for automatically registering the relationship between the ID information of each transmitter and the position of each wheel.
The tire pressure monitoring apparatus of the patent literature 1 detects that a wheel is at a predetermined rotational position based on an acceleration detection signal of an acceleration sensor provided in the transmitter as well as detects a rotational position of a wheel when the receiver receives a radio signal from a transmitter. The tire pressure monitoring apparatus specifies the position of the wheel by monitoring a change in a relative angle between the rotational position detected by the transmitter and the rotational position of the wheel when the receiver receives the radio signal from the transmitter.
Namely, the change of the relative angle between the rotational position of the wheel detected by a wheel-side unit fixed to the wheel and the rotational position of the wheel detected by a body-side unit fixed to the vehicle body is monitored based on a deviation of predetermined number of data. The position of the wheel is specified by determining whether a variation of the change of the relative angle relative to an initial value is over an acceptable value.
In the method described in the patent literature 1, the radio signal is transmitted when the wheel is at the predetermined rotational position. However, there is a position where the radio signal is likely to be difficult to reach the body-side unit, and such a position is so-called Null. Therefore, if the radio signal is transmitted when the rotational position of the wheel is at the Null, the radio signal is less likely to reach the body-side unit, even if the radio signal is transmitted many times. In such a case, it takes time to detect the position of the wheel, or it is difficult to detect the position of the wheel.
Patent literature 1: Japanese Patent Application Publication No. 2010-122023